Simplified Vowel Synthesis Interface - from the University of Delaware. Synthesize non-nasalized monopthongal English vowels using the Klatt synthesizer. Set F0, F1, F2, and duration in ms yourself, and see what you come up with. Links to more complex CV synthesis and general synthesis interfaces.

Zona Land - by Ed Zobel. Excellent interactive tutorials on the physics of waves (with links to other physics topics); I find them to be well-matched companions to D. B. Fry's The Physics of Speech (Cambridge). Example of things you can do at this site: draw two waves yourself then watch how they interfere with each other. VRML plug-in required for some parts. Highly recommended.

The Physics Classroom: Sound Waves and Music - from Glenbrook South High School. A good physics tutorial site with units on waves and decibels; this has dense and very instructive explanatory texts, along with animated graphics.

Standing Waves and Sound - from Concordia College. Samples of some audible frequencies (100 to 5,000 Hz), along with other information on waves.